A few black cabs will be zero-emitters. For the first hour or so each day, anyway.

There are also different kinds of fuel cells and, especially, different fuels on which they can run: most of the more practical fuel options are hydrocarbons and involve CO2 emissions, though a fuel cell running on troublesome hydrogen emits only water vapour. (Bear in mind though that the hydrogen will probably be made by reforming natural gas and so emitting carbon.)

Another one the mayor's people evidently forgot or didn't know about is hydrogen internal combustion: it's actually fairly simple to run a normal car engine on hydrogen from a special tank instead of petrol, though it does involve some minor emissions apart from water vapour and it uses rather a lot of hydrogen. This means that one will normally need to store the stuff as a cryogenic liquid, which leads to the issue of the liquefied gas boiling off from its tank. Thus it's unwise to park such a vehicle in a confined space – or to leave it parked and fuelled up for too long, as all the fuel will be gone in a matter of days.

Battery-only cars would make terrible taxicabs: with technology likely to be available in a taxi by 2020, they would spend more time charging up than they would earning their keep, and even if fast-charging batteries and numerous industrial power outlets could be deployed that fast at reasonable cost, the brutal charge-discharge cycle implicit in taxi work would require prohibitively frequent replacements.

Hydrogen fuel cells might work better, but hydrogen supplies would be an issue and fuel cells are punishingly expensive. A black cab is already very pricey even using mainstream technology, and an affordable hydrogen fuel-cell one by 2020 – even with subsidies – is probably a pipedream.

The likeliest technology for the "zero emissions capable" taxi of 2020 is, of course, plug-in hybrid – it's no accident that this is the tech that General Motors is banking on with the Volt.

The only snag with this is that a plug-in hybrid can only operate as a zero-emissions vehicle by driving small distances each day. This is quite feasible for a typical commuter, but hardly so in the case of a taxi driver. The first couple of dozen miles driven will be in zero-emission battery mode, and then for the rest of the day the engine will provide all the energy: the taxi will have become basically just another hybrid.

Nothing wrong with that, of course – hybrid has much lower emissions than a typical present day diesel cab – but the use of the term "zero emission" is at best ignorant and at worst misleading. The mayor might well expect to be punished by the media and the voters for making such a misleading pledge.

But in fact it wasn't highlighted by the mayor's office: the announcement of the strategy focused rather on simpler, nearer-term plans such as phasing out the oldest diesel cabs and introduction of twice-yearly inspections in place of annual – which latter should ensure that cabs are better adjusted and emit less pollution. Rather than CO2, it is emissions of NOx and particulates (eg diesel soot) which are a major issue in London.

In any case, Mayor Boris is probably safe from informed or accurate media analysis of his plans – even supposing he is still in the Mayor's office in 2020 to account for himself. The fabulous soaraway Guardian, having read the strategy, gaily assures us that "all London black cabs will be electric by 2010, says Boris Johnson" (picture caption and, less obviously wrongly, headline).

Quite apart from the fact that there are only a couple of weeks of 2010 left (whoops), what Mayor Boris actually has said is something more like "a small proportion of London black cabs will be electric for a small part of the day starting in 2020".